Biphasic changes in hemoglobin A_1c concentrations during normal human pregnancy

Analyses of hemoglobin A_1c concentrations were performed throughout gestation in 377 nondiabetic women. We observed significant biphasic changes in hemoglobin A_1c concentrations, with an initial gradual decline to a nadir level at 24 weeks, followed by a subsequent slow reascension to peak near term. All these changes fell within the usual range of normal values for hemoglobin A_1c. Values for plasma glucose estimated 1 hour after a 50 gm oral glucose load in 1,756 normal gravid women showed similar biphasic excursions, with the nadir occurring 4 weeks earlier, i.e., at 20 weeks' gestation. We conclude that the small but significant changes in hemoglobin A_1c during the course of normal gestation reflect, with an appropriate displacement in time, the biphasic alterations in mean blood sugar that characterize the sequential changes in glucoregulation during normal pregnancy.